Date Calculator
Use this date calculator to quickly calculate the days between two dates. Whether you want to use it to work out how many days are left until the summer holidays, check precisely how old you are (in years, days, and even seconds), or plan how many books you're able to read in a given period, the date difference calculator will come in handy. Using it, you can easily find out which date is 45 days from today, what day of the year it is, and calculate the time between two dates in years, months, weeks, days, hours, minutes, and seconds.
How to use the date/duration calculator
 To count down to an event:

Select today as a start date.

Select the date you are waiting for in the "end date" field.

The time difference will be the time left until the date. You can display it in different units.
 To check duration (number of days between two dates):

Input the date that begins a given period as the start date e.g., the first day of a school year.

Input the date marking the end of a period e.g., the last day of school.

The time difference will be the time between dates e.g., how many days the school year lasted.
 To add or subtract days:

Choose a start date from which you're going to add or subtract days. For example, if you're just starting an intense threemonth tap dancing course and want to know what day will be 90 days from today, input today's date.

To add days, enter the number of days (or other time units) into the "time difference" field. If you want to subtract them, put a minus before a number, e.g., "10" days. In our example, we add three months, so change the unit to "months" and enter "3".

The end date will be the date 3 months from the start date (or before it if you subtracted days). You can see the date on which you will (hopefully) have become a master at tap dancing.
If you need to count only working days, or just some weekdays, use the day counter, this date to date calculator unfortunately cannot help you.
How to calculate days between two dates by yourself
To calculate the days between two dates, say 4.09.1999 and 2.01.2003, you need to:
 Calculate the number of full years between the two:

Add as many full years as possible to the earlier date in such a way that you don't go past the later date. In our example, you can go as far as 4.09.2002. If you added one more year, we would have 4.09.2003, which is later than 2.01.2003.
So we went from 4.09.1999 to 4.09.2002. Calculate how many years are between those dates by subtracting the later year from the earlier one:
2002  1999 = 3

Count how many days were in those years. Take leap years into account if necessary. In our example, we have one leap year  2000:
Y = 2 * 365 + 366
Y = 1096
 Calculate the full months difference left:

You need to know how many days there are in each month. In case you don't  February has 28 (29 in a leap year) days, September, April, June, and November have 30 days each, and the rest have 31.

From 4.09.2002 to 2.01.2003 there are three full months  October, November, and December. Multiply each month by the corresponding number of days and add them together:
M = 31 + 30 + 31
M = 92
 Calculate the remaining days:

Calculate the remaining days left in the start date's month  September in our example (how many days from 4.09 until the end of the month). You do this by subtracting the date (here 4) from the total number of days in the month:
31  4 = 27

Add the result to the day of the month the end date is (just the number part of the date, here 2):
D = 27 + 2
D = 29

Add the results (Y, M, D from points 13) to get the number of days between 4.09.1999 and 2.01.2003:
Y + M + D = 1096 + 92 + 29
Y + M + D = 1217

If you don't want to include the end date, subtract 1 from the result:
1217  1 = 1216
It may seem quite complicated, but luckily you can just input the dates into the date calculator and bask in the luxury of having a computer work out the problem for you.
Common applications  where you may find yourself counting the days between dates
You may find the date calculator useful if you want to check:

how many days are left until a deadline or exam.

events duration, e.g., how long the school year is.

how old you are in years, months, days, or seconds.

when you'll be (or were) one billion seconds old.

how many days are left until a longed awaited day  wedding, vacation, the birthday of your pet iguana, etc.

how many days have passed since an event.

which day of the week it will be on a given day.

your due date or conception date.

the start of your period or when you will ovulate.

add or subtract the number of days/weeks/months/years, e.g., you set a sixweek training plan and want to know which day will be six weeks from the day you start.
The short history of conventional time
In prehistoric times, long before such ingenious inventions as a date to date calculator, most peoples around the world would orient their time around the lunar phases and the changes of the seasons. The word calendar derives from the Latin calendae, which meant the first day of the Roman month, marked by a new moon in the sky. Calare meant "to announce", or "to call out." The priests observed the new moons from the Capitoline hill, and when they saw it they would announce the number of days until the next month. On that day, the debts noted in an accounting book called the kalendaria had to be paid off.
After Julius Caesar seized power, he decided to reform the Roman calendar in 46 B.C. As the Roman Lunar Calendar only had 355 days, it would drift out of sync with the seasons if the Pontifex Maximus (also Caesar) didn't monitor it and add days where appropriate. Since he had been fighting a civil war, the Pontifex Maximus had not been doing his duty, and so by 46 B.C. it was out by about three months.
Caesar had spent some time in Egypt and was inspired by their solar calendar. He hired an Egyptian astronomer, Sosigenes of Alexandria, and together they built their own Roman solar calendar. Whereas the Egyptian calendar had twelve 30day months, with five extra festival days to be allotted, the Roman calendar would spread these 5 days out between the months, while also taking two days from February as it was considered an unlucky month. They were also aware that the year was actually 365.25 days, and also added an extra leap day in February every four years. The effect of this was that the calendar would now run on autopilot  festival days could no longer be added willynilly by politicians for some nefarious purpose  time would tread on independent of any man's machinations.
The new Roman (or Julian) calendar was not perfect, however. Using the most hightech techniques of the time, it created a 365.25 day year. Alas, it is actually 365.24219 days. This caused the calendar to gain a day on the solar year every 128 years. This slight drift really, really annoyed the clergy, who wanted Easter to be celebrated at the same time of year as the early church. This noise grew and grew, until eventually it reached the ears of the Pope. Pope Gregory XIII, to be exact. He hired some mathematicians to look into the issue.
The mathematicians reported back that by being off by a little over 11 minutes, the calendar had three unneeded year days every 400 years. To right this wrong, the mathematicians added in a clause that if the year was a multiple of 100, the leap day would be skipped, unless of course the year was also a multiple of 400, in which case the leap day would be kept. This would mean that the calendar would only be out of date after 3216 years. The Pope issued a Papal Bull, and Catholic Europe adopted the calendar by the end of 1582.
Protestant countries initially objected to adopting the calendar. Great Britain and its colonies wouldn't use it until 1752, and the last country in Europe was Greece, which followed the changes in 1923.
Nowadays, the Gregorian Calendar is used in most countries around the world. The few exceptions include Afghanistan, Iran, Ethiopia, and Nepal.